Until the development of copper interconnect technology, copper was not found in wastewater from the production of multilayer microchips by the semiconductor industry. Copper is now being used as a replacement for aluminum and tungsten because of its lower electrical resistivity. In the process of manufacturing multilayer chips, there are many steps including the deposition of the dielectric layer (silicon dioxide or low-k polymeric), etching the interconnect pattern (trenches and vias) into the dielectric layer, deposition of copper metal into the trenches and vias, and chemical mechanical polishing (CMP) to remove excess copper and create a level surface prior to creation of the next layer of the chip. The CMP step is accomplished by the use of polishing pads and proprietary polishing slurries which contain abrasive solids such as alumina, oxidants such as peroxide, chelants such as citrate, and other additives such as corrosion inhibitors. Therefore, the resulting wastewater contains chelated copper, oxidants, additives and abrasive solids. The presence of the abrasive solids at concentrations of 200-5000 mg/L makes this wastewater different than the typical metal-containing wastewater from electroplating operations.
Wastewater, from the metal-CMP step of microchip manufacture, can vary widely depending on the original slurry composition and the CMP tool design and operating parameters. The slurry is diluted by rinse water during polishing. The amount of rinse water used determines metal and abrasive solids levels in the wastewater.
Several polymer chemistries have been used to treat wastewater containing transition metal complexes, such as copper-ethylenediaminetetraacetic acid (EDTA), resulting in the precipitation of the metal-polymer solids. These polymer chemistries contain an amine functionality that can be reacted with carbon disulfide to form dithiocarbamate (DTC) functionalities on a polymer backbone. One such polymer is a carbon disulfide modified ethylenedichloride-ammonia condensation polymer, as described in U.S. Pat. No. 5,164,095. The polymer disclosed in the '095 patent is a low molecular weight, highly branched material. Other polymer backbones suitable for modification with carbon disulfide include the polyethylenimine (PEI) polymer described in U.S. Pat. No. 5,387,365, the epichlorohydrin and multifunctional amine condensation polymer disclosed in U.S. Pat. Nos. 4,670,160 and 5,500,133 and the polyallylamine polymer taught in EP 0 581 164 A1. Despite these known polymer chemistries, however, there is still a need for new polymers containing DTC functionalities which effectively treat wastewater and possess other desirable attributes such as low levels of product impurities (e.g., sodium sulfide, which is toxic and foul smelling), relative ease of manufacture (e.g., to avoid the gaseous ammonia in ethylenedichloride and azeridine in PEI) and improved solids/liquid separation characteristics.
U.S. Pat. No. 5,346,627 describes the use of polymers containing DTC functionalities for the treatment of soluble metals and subsequent removal of precipitated solids in a filtering device, including a microfilter. However, the '627 patent does not describe the use of such polymers for simultaneously precipitating metal ions from semiconductor wastewater and enhancing microfilter operation.
There are two parameters of concern during microfilter operation. One is flux which is defined as the flow of purified water divided by the membrane area. In microfiltration, one way of expressing this is as gallons of pure water per square foot of membrane area per day or GFD. Another way of expressing this is permeability, which is flux divided by trans membrane pressure (TMP). Permeability is essentially "normalized" flux which takes into account changes in system pressures. Both flux and permeability are used to describe the passage of water through the membrane, however, they are not interchangeable. The other parameter of concern is solids passage. Generally speaking, the purpose of the microfilter is to separate solids from liquids in a bulk solution. Because the microfilter has a distinct cutoff size (ranging from approximately 0.1 to 5.0 microns depending on method of manufacture), only particles larger than the cutoff are retained, in theory. However, as is the case with all membrane processes, a percentage of the total solids will pass through the membrane. Therefore, as the initial concentration of the feed water increases, so too will the absolute value of the solids content of the permeate water. The percentage of materials which pass through the filter, however, remains largely the same unless membrane damage has occurred.
In many operations, microfiltration is used to perform the functions of a clarifier and a media filter. This is because it has a small footprint and performs these operations more quickly than conventional technology. It is important, therefore, to keep the microfilter in good operating condition. A major problem with microfiltration is that the filters can become fouled or plugged with fine solids. This causes the flux to decrease in the unit, and it must be taken off line to be cleaned. Microfilter operation can be enhanced by using additives which result in higher flux values and longer times between occurrences of fouling. Flux enhancement is desirable because it decreases the amount of time during which the equipment is out of service, thereby increasing its overall efficiency.
Accordingly, it would be desirable to provide a new water-soluble polymer containing DTC functionalities which effectively treats metals-contaminated wastewater and possesses other desirable properties, such as a low level of product impurities, relative ease of manufacture and improved solids/liquid separation characteristics. It would also be desirable to provide a method for the use of such a polymer to simultaneously precipitate soluble heavy metal ions from semiconductor wastewater containing abrasive solids and enhance microfilter operation.